In process measurements and automation technology, for the measurement of physical parameters, such as e.g. mass flow, density and/or viscosity of a medium flowing in a pipeline, often such inline measuring devices are used, which, by means of a measurement transducer of vibration-type, through which the medium flows, and a measuring device electronics connected thereto, effect reaction forces in the medium, forces such as e.g. Coriolis forces corresponding with the mass flow, inertial forces corresponding with the density of the medium and/or frictional forces corresponding with the viscosity of the medium, and which produce, derived from these forces, a measurement signal representing the respective mass flow, viscosity and/or density of the medium.
Mostly, the measuring device electronics is also so designed that during operation the inline measuring device can exchange measurement and/or other operational data via a data transmission system with a measured value processing unit, for example, a programmable logic controller (PLC), a personal computer and/or a workstation. Furthermore, the measuring device electronics is so designed that it can be fed by an external energy, or power, supply. Therefore, the measuring device electronics has, additionally, a corresponding communications interface e.g. for transmitting the measured data to the aforementioned programmable logic controller and/or superordinated process control system. Serving as transmission systems are usually fieldbus systems, for example Profibus or Foundation Fieldbus systems, etc., with which both the energy supply and the data communications can be implemented.
Measurement transducers of the described kind, especially those embodied as Coriolis mass flow meters or Coriolis mass flow/density meters are described comprehensively and in detail e.g. in WO-A 05/111550, WO-A 04/083785, WO-A 04/038341, WO-A 03/076879, WO-A 03/027616, WO-A 03/021202, WO-A 02/088641, WO-A 01/33174, WO-A 00/57141, WO-A 98/07009, U.S. Pat. Nos. 6,711,958, 6,666,098, 6,526,839, 6,412,354, 6,308,580, 6,301,973, 6,092,429, 5,918,285, 5,796,011, 5,370,002, 5,301,557, 5,226,330, 5,024,104, 4,876,898, EP-A 553 939, EP-A 1 001 254, EP-A 1 448 956 or EP-A 1 421 349. For conveying the, at least at times, flowing medium, the measurement transducers include in each case at least one transducer tube held in a mostly thick walled, especially tubular and/or beam-like, support cylinder or support frame. The transducer tube is caused to vibrate during operation for producing the aforementioned reaction forces, driven by an electromechanical exciter mechanism. For registering vibrations of the transducer tube, especially on its inlet side and its outlet side, and for producing at least one oscillation measurement signal representing these, such measurement transducers include, additionally, in each case, a sensor arrangement reacting to movements, and thus also to mechanical oscillations, of the transducer tube.
During operation, the above described inner oscillation system of the measurement transducer formed by the at least one transducer tube, the medium conveyed at least instantaneously therein, and at least in part by the exciter mechanism and the sensor arrangement is excited by means of the electromechanical exciter mechanism, at least at times, into a wanted oscillation mode to execute mechanical oscillations at least one dominant, wanted, oscillation frequency. These oscillations in the so-called wanted mode are, especially in the case of use of the measurement transducer as a Coriolis mass flow and/or density meter, developed at least in part as lateral oscillations. The wanted oscillation frequency is, in such case, usually a natural, instantaneous, resonance frequency of the inner oscillation system, which in turn depends both on size, shape and material of the transducer tube, as well as also on an instantaneous density of the medium; as required, the wanted oscillation frequency can also be influenced significantly by an instantaneous viscosity of the medium. As a result of fluctuating density of the medium to be measured and/or as a result of medium changes effected during operation, the wanted oscillation frequency is variable during operation of the measurement transducer, according to its nature, at least within a calibrated and, as a result, predetermined oscillation frequency band, which correspondingly has a predetermined lower, and a predetermined upper, limit frequency. The inner oscillation system of the measurement transducer formed by the at least one transducer tube, as well as the exciter mechanism and the sensor arrangement is furthermore usually placed in a transducer housing having as integral component the support frame or support cylinder as the case may be. Transducer housings appropriately suitable for measurement transducers of vibration-type are described for example in WO-A 03/076879, WO-A 03/021202, WO-A 01/65213, WO-A 00/57141, U.S. Pat. Nos. 6,776,052, 6,711,958, 6,044,715, 5,301,557 or EP-A 1 001 254.
Oscillation exciters and oscillation sensors are components of the measurement transducer and are, for the accuracy of measurement, of decisive importance, both with respect to their mechanical, as well as also with respect to their electrical, ability to withstand disturbances. The more so, since the exciter mechanism and sensor arrangement are mostly directly exposed, based on the operating principles of measurement transducers of the described kind, to oscillations of the transducer tube. As a result of this, they are subjected not only to increased mechanical loads but also, due to the movement of lines conveying electrical current, mostly also to considerable electrical disturbing influences. Moreover, as discussed for example in U.S. Pat. No. 4,876,898 or in WO-A 04/083785, high, and/or strongly fluctuating, operating temperatures can have a considerable influence on the accuracy of measurement, especially, however, also on the mechanical integrity of exciter mechanism and/or sensor arrangement.